Microbeam irradiate nuclear and/or cytoplasmic regions of individual human cells with single or spatially defined multiple a-particles and determine effects on cell cycle progression and Of early and late chromosome specific changes. These endpoints will be assessed using a tagged monoclonal antibody against bromodeoxouridine substituted DNA and by whole chromosome specific probes hybridized against interphase nuclei (chromosomal domains) and mitotic spreads (individual chromosome pairs). Probe localization will be by single cell fluorescent in situ hybridization.